The major advantage of this technique is that multi-bounce end-pumped lasers or amplifiers can be scaled up to high powers (&gt;100 Watts) while maintaining fundamental mode (TEM.sub.00) operation and high optical conversion efficiency. End-pumped lasers offer many advantages over side-pumped lasers in efficiency and beam quality. This is true if all of the pump energy can be imaged inside the fundamental mode radius of the solid state laser and if the gain medium is long enough to absorb a large fraction of the pump light. A disadvantage of an end-pump laser is that a gradient in the index of refraction is produced within the laser mode, due to nonuniform heating of the laser gain material. This problem is addressed in S. Kubota et. al., "Thermal aberration analysis of a laser-diode-pumped Nd:YAG laser", Technical Digest of Lens Design Conference", Manuscript No. 1354-17, (1990) and by M. E. Innocenzi, et. al., "Thermal Modelling of Continuous-wave End-pumped Solid-state Lasers", Appl. Phys. Lett., vol. 56, 1831 (1990). The effect of a nonuniform refractive index on the laser mode is similar to the effect of a lens positioned close to the pump light beam entrance point. With the availability of high power AlGaAs laser diodes, high power at high power density is now possible at a single pump point, but thermal lensing of the laser gain material must be compensated for if single point pumping is to be used.
U. O. Farrukh et al, in "An Analysis of the Temperature Distribution in Finite Length Solid-state Laser Rods", I.E.E.E. Jour. Quant. Electr., vol. 24, 2253-2263 (1988), have theoretically analyzed the temperature distribution in an end-pumped solid state laser rod of finite length for single pulse heating and for continuous or long pulse heating, assuming that cooling from each rod surface is linear and that temperature excursions from ambient are modest. This analysis also assumes that isotropic material behavior prevails in the rod, which is not true when thermal lensing develops. M. E. Innocenzi et al, ibid, extend the Farrukh et al analysis to situations where the rod surface is convectively cooled and is in contact with an isothermal heat sink or source. These authors compute the single pass, optical phase change associated with refractive index changes and find that, in one representative example, thermal variations in this index account for 74 percent of the thermally induced optical focusing.
Keirstead and Baer, in "Ten Watt TEM.sub.00 output from a diode-pumped, solid state laser", Abstract CFC3, Conference on Lasers and Electro-optics, 1991, discuss use of a tightly folded resonator ("TFR") laser oscillator design in which light from three laser diode bars is coupled into an optical cavity that is defined by a plurality of reflective surfaces. All reflective surfaces but one of the TFR are planar, and Keirstead et al report a 32 percent pumping efficiency for an Nd:YAG laser driven at 27 Watts. However, this configuration does not permit easy scale-up of the input power, and thus the output power, by use of additional reflectors because thermal lensing, thermal depolarization and other maladies that arise at higher laser power levels limit such scale-up. Laser mode spot size mismatch and other problems arise where a larger number of reflectors are used.
Seelert, Skrlac and Kortz, in "One Watt single frequency diode pumped Nd:YAG laser system", Abstract MF5-1, Advanced Solid State Laser Conference, Hilton Head, SC, March 1991, p. 104, discuss use of a multi-stage, end pumped Nd:YAG laser gain system to produce an amplifier ("EDDA") that produces an light beam with output power 600-1,000 milliwatts. Each stage of the EDDA includes an input lens, a prism with first and second fully reflecting surfaces, a laser gain medium to receive an input light beam from the first reflecting surface of the prism at a first end of the medium, and to receive an end pump light beam from a second end of the medium, a planar reflecting at the second end of the laser gain medium to reflect the amplified input light beam to the second reflecting surface of the prism, and an output lens to receive the amplified input light beam from the second reflecting surface of the prism. The input and output lenses serve to collimate or refocus the light beam from stage to stage, and all reflecting surfaces are planar. Development of a thermal lens within each laser gain medium is not explicitly compensated for within each stage, and the amplifier output power is limited to a modest value of about one or a few Watts.
What is needed is an optical configuration that allows straightforward scaling of laser output power to higher values without the limitations imposed by the presence of thermal lensing and other thermal problems that appear at the higher laser power levels.